1. Field of the Invention
The present invention relates to image forming apparatuses, such as a copier, a multifunctional peripheral device, and a printer. Particularly, the present invention relates to an image forming apparatus and an image forming method of electrophotography using photoconductor.
2. Description of the Related Art
A copier and a laser beam printer of the electrophotography are known as image forming apparatuses of a high speed and a high definition digital system. In recent years, such an image forming apparatus is spreading out not only into an office or a home but into a commercial print market, and higher definition of an image forming apparatus is requested.
There are various important image quality items, such as color reproducibility, gradation reproducibility, dot-line reproducibility, and graininess, when considering characteristic of an output image quality of an image forming apparatus. The graininess among the image quality items is inconsistency in densities (variation) of dots that constitute a halftone image. The graininess is a particularly important image quality item, because it has large effect on the reproducibility of photo image quality and the image appears to be rough when the graininess deteriorates.
Although the granularity is initially determined by an initial condition of the image formation in the electrophotography system, it varies according to environmental variation or time degradation of configuration members. Therefore, the method of controlling the sequential variation of the granularity of an image forming apparatus is proposed (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2004-101564 (JP2004-101564A)).
The technique described in JP2004-101564A reads information about the inconsistency in density of a developed toner image by a light reflex type sensor, and calculates an index showing the graininess. Then, when at least one of a variation of the inconsistency in density or a variation of the density of an image exceeds a tolerance level, the technique decreases the inconsistency in density by combining a first control factor that improves the graininess with increasing toner deposit and a second control factor that improves the graininess with increasing toner deposit.
For example, as described in FIG. 20 of JP2004-101564A, electric potential of an electrostatic latent image area (developing bias) is changed as the first control factor, and linear velocity ratio of a developing roller to a photoconductor is changed as the second control factor. Although an average toner deposit increases as the developing bias increases, the graininess deteriorates simultaneously. In other words, when the developing bias becomes small, the average toner deposit decreases and the graininess improves simultaneously. Although the average toner deposit increases as the linear velocity ratio of the developing roller increases, the graininess improves. Therefore, JP2004-101564A describes that the average toner deposit and the graininess (the inconsistency in density of an image) are controllable independently by controlling the developing bias and the linear velocity ratio of the developing roller suitably.
Incidentally, the conventional image forming apparatus of the electrophotography system forms an electrostatic latent image on a uniformly electrified photoconductor by exposing selectively based on image data, and forms a toner image on the photoconductor by developing the electrostatic latent image with toner. The toner image formed on the photoconductor is transferred to a recording sheet, and the image is formed by fixing the toner image to the recording sheet in a fixing process after transferring.
Such an image forming apparatus removes the toner that is not transferred and remains on the photoconductor by contacting a cleaning member, such as a cleaning blade, to the photoconductor in order to scrape the toner off after image formation. Therefore, friction between the photoconductor and such a cleaning member over the long term chips the surface of the photoconductor gradually, which reduces film thickness of the photoconductor (photoconductor thickness).
According to investigation, the inventors of the present invention found a phenomenon in which the graininess of an outputted image deteriorated by reduction of the photoconductor thickness. Since the technique described in the patent publication does not take the photoconductor thickness into consideration as a control factor as mentioned above, deterioration of the graininess by reduction in the photoconductor thickness cannot be controlled. Since the technique described in the patent publication controls the two control factors simultaneously or sequentially, there is a problem of taking time until optimizing the graininess.